Photovoltaic devices, photoelectric conversion devices or solar cells are devices which convert light, especially sun light, into direct current (DC) electrical power. For low-cost mass production thin film solar cells are being of interest since they allow using glass, glass ceramics or other rigid or flexible substrates as a base material, i.e. a substrate, instead of crystalline or polycrystalline silicon. The solar cell structure, i.e. the layer sequence responsible for or capable of the photovoltaic effect, is being deposited in thin layers.
In industrial scale production of photovoltaic cells, the optimization of procedural features in a manufacturing sequence is of major importance. Reliability, throughput, and/or energy efficiency directly affects the price of a product manufactured. Many deposition processors for manufacturing of photovoltaic cells take place at elevated process temperatures of 200° C. or more. Handling, transporting and/or examining of substrates however often require ambient temperatures. It is therefore possible that a substrate needs to undergo several heating/cooling cycles during the manufacturing process. Often such heating/cooling is performed during pump/down or venting cycles in a load lock, i.e. an evacuable enclosure that is operatively connected to a vacuum processing system, and which allows for accessing the vacuum processing system via sealable openings or doors.
Thus, rapid heating and/or cooling of substrate materials directly influences overall systems throughput, which is an essential criterion for a manufacturer due to the expensive hardware required. Consequently, system throughput is directly related to the thermal efficiency of the load lock. Thermal efficiency further depends on the heating rate of the heating elements, such like lamps, and on the substrate uniformity, which both influence the thin film layer quality.
Referring to the manufacturing process of thin film photovoltaic cells, the load lock temperature treatment of substrate material is a prerequisite to deposit high quality ZnO layers for front and/or back electrodes in a thin film application such as DVD, CVD, PECVD, APCVD, or MOCVD. Various ways of temperatures treatment are presently used to heat a base material from ambient temperature to process temperature.
Heating technologies known in the art use short, medium, and/or long wave lamps, with filaments manufactured from tungsten, carbon, graphite, or alike. Each lamp assembly includes a transmissive tube housing the filament. The housings prevent the lamp base materials from oxidation and increase the longevity. Lamp housings are necessary to protect the filament from e.g. oxidizing in ambient atmosphere. On the other hand, the lamp housings narrow the emitted radiation by a certain fraction. This filter effect directly influences the thermal efficiency. Consequently, the efficiency of lamp-based heating systems directly affects the throughput and results in decreased market competiveness for manufacturer of such systems.
Improved, more transmissive lamp housings known from prior art have a certain mass. In general, mass functions as heat reservoir and decreases the system dynamic impairing the controllability of the heating system. Decreasing the temperature controllability and/or stability means that temperature uniformity fluctuates by a certain value. Further, a cooling mechanism needs to be installed to counteract temperature variations, which results in an increased investment volume. In addition to the before described filtration effect, the applied electrical power to the lamp has to be increased to such an extent for compensating the emitted light spectrum loss, which again results in increased energy consumption and subsequently low system energy efficiency. Further, in order to heat extended two-dimensional substrates, several tube-shaped lamps have to be arranged side-by-side to form an array of heating elements. However, this pattern of heating elements will be visible in the heat distribution of the substrate, i.e. has a negative impact as well.